Astronomers have traced a mysterious type of repeating cosmic signal to an unusual pair of stars, providing the strongest evidence yet for one of astronomy’s most puzzling phenomena.
The discovery was made by an international research team led by scientists from the University of Sydney, using CSIRO’s ASKAP radio telescope. Their discovery pinpoints the origin of an unusual class of objects known as long-period radio transients. It’s a mysterious radio burst that has puzzled astronomers ever since it was first detected in a handful of locations throughout the Milky Way.
The result is natural astronomy.
Lead author Kovi Rose, a PhD student at the University of Sydney’s School of Physics and CSIRO, said the research team was finally able to link one of these mysterious signals to a specific type of star system.
“For the first time, we have identified the source of these signals and confirmed that the source is a ‘cataclysmic variable,’ an accreting white dwarf,” Rose said.
“Long-period radio transients have puzzled astronomers for years,” Rose said. “We only found about a dozen, and their origin was unknown. Now we can show that one source of these transients comes from a white dwarf star that is actively pulling material from its companion star.”
Rare white dwarf star system revealed
The newly identified star system, known as ASKAP J1745-5051, consists of a white dwarf star and a red dwarf star confined in very close orbits. A white dwarf is a dense remnant of a dead star, about the same size as Earth but with a mass comparable to the Sun. Its companion star is a much larger, but less dense red dwarf star with about one-tenth the mass of the Sun.
The two stars orbit each other in just over an hour.
When a white dwarf pulls gas from its companion star, the material heats up and emits X-rays. At the same time, interactions between the stars’ magnetic fields produce powerful radio bursts. These emissions are repeated in regular cycles every 1.4 hours.
“All of these emissions are related to the orbital motion of this system,” Rose said. “But interestingly, the radio and X-ray signals do not peak at the same time, indicating that they are generated in different regions of the system.”
The researchers found that radio waves are likely to be generated where the star’s magnetic field collides and interacts with a stream of charged material flowing toward the white dwarf. This process produces a tightly focused burst of radiation that travels throughout space.
Solving the mysteries of long-period radio transient phenomena
When long-period radio transients were first discovered, many astronomers suspected they were unusually slowly rotating neutron stars known as pulsars. However, existing models suggest that slowly rotating neutron stars should be unable to generate these signals.
New findings support an alternative explanation. At least some long-period radio transients appear to occur in binary systems involving white dwarfs.
Professor Murphy, chair of the University of Sydney’s School of Physics and principal investigator at the ARC Gravitational Wave Discovery Center of Excellence (OzGrav), said: “There have been several similar objects previously associated with binary star systems, but this is the first time that we can clearly see both the star and the actual accretion process.”
This system is also the second known long-period radio transient to produce conventional X-rays. This is the first time scientists have pinpointed the cause of periodic behavior.
rosetta stone in space
Researchers believe ASKAP J1745−5051 could become an important reference object for understanding other mysterious radio transients.
The system was discovered using ASKAP, a radio telescope owned and operated by Australia’s national science agency, CSIRO. ASKAP combines a wide field of view, high resolution, and excellent sensitivity, making it particularly effective at detecting anomalous signals that may otherwise go unnoticed.
“This system provides a way to decipher these signals. It may help determine whether other long-period transients are more like pulsars or white dwarf systems that act like the Rosetta Stone of stars,” Rose said, referring to an archaeological object discovered in Egypt that helped translate ancient hieroglyphics.
The system not only helps explain mysterious radio signals, but also provides scientists with a rare opportunity to study extreme physical conditions that cannot be reproduced in laboratories on Earth.
“These systems are nature’s laboratories,” Rose says. “They allow us to test our understanding of how matter behaves under strong magnetic fields and strong gravitational forces.”
Future observation schedule
The research team plans to continue studying this system using radio, optical, and X-ray telescopes. By combining observations across different wavelengths, they hope to better understand how these signals are generated and whether similar mechanisms can explain a broader range of long-period radio transients.
“Each new discovery helps put the picture together,” Rose said. “We are just beginning to understand this new kind of cosmic event.”
The international collaboration included researchers from Australia, the United States, China, Canada, Spain, and Israel. Observations were carried out using CSIRO’s ASKAP and Australian Telescope Compact Arrays in Australia, the MeerKAT radio telescope in South Africa, SOAR and Magellan Optical Telescopes in Chile, and the Swift (UV/X-ray) and Einstein Probe (X-ray) observatories in space.
The authors reported no competing interests. Funding for this research was provided by the Australian Research Council Gravitational Wave Discovery Center of Excellence (OzGrav), NASA, the Alfred P. Sloan Foundation, the Harry Messel Professorial Physics Fund Research Fellowship, the European Research Council, and the China Scholarship Council.
